Method of making an electric fuse



Dec. 6, 1966 J, GAIA 3,289,278

METHOD OF MAKING AN ELECTRIC FUSE Original Filed Sept. 19, 1960 United States Patent ffire De a, 196%;

l mains. (Cl. ze -teas) This is a division of application Serial No. 56,884, filed September 19, 1960, now Patent No. 3,227,841.

This invention relates to improvements in protectors for electric circuits. More particularly, this invention relates to improvements in small enclosed electric fuses.

It is therefore an object of the present invention to provide an improved small enclosed electric fuse.

There is a progressive demand for reductions in the sizes of electrical components; and that demand has been extended to the art of electrical fuses. However it is not always easy to reduce the sizes of electrical fuses, because an electric fuse must have an arc gap that is large enough to clear any arcs that form as the fuse opens the circuit. Yet, it would be desirable to provide extremely small enclosed electric fuses. The present invention provides such fuses; and it is therefore an object of the present invention to provide and extremely small enclosed electric fuse.

The fuse provided by the present invention can be small enough to fit within a housing that is approximately one quarter of an inch long and approximately One quarte of an inch in diameter; and yet th t fuse can he used to open circuits carrying two hundred and forty volts. The present invention attains this startling result by connecting the fusible conductor of the fuse to terminals that are circular in cross section and that are disposed immediately adjacent the inner surface of the housing and by making the wall of that housing thin. Those terminals serve as fixed arc horns which are spaced apar a distance greater than one half the inner diameter of the housing. Further, those terminals are massive relative to the fusible conductor and will thus cool any are that forms as the fusible conductor opens the circuit. In addition, the fusible conductor extends transversely of the axes of those terminals so the convex surfaces of those terminals can simulate spherical arc horns and thereby facilitate the quenching of any are that forms as the fusible conductor opens the circuit. It is therefore an object of the present invention to provide an electric fuse which has its fusible conductor connected to, and extending transversely of the axes of, terminals which are circular in cross section and which are disposed immediately adjacent the inner surface of a thin-walled housing.

It is usually desirable to provide a way of readily determining whether the fusible conductor of a fuse is intact or has fused to open the circuit; but with an extremely small enclosed electric fuse it is particularly desirable to provide a way of readily determining whether the fusible conductor of that fuse is intact or has fused to open the circuit. Such a fuse could not be tested by an ohmmeter, because the current supplied by the battery of the ohrnmeter could cause the fusible conductor to fuse. While that fuse could be removed and a substitute fuse could be inserted in its place, such removal and insertion is time-consuming and would subject the fuse to needless stresses and strains. Further, it would be difficult to know whether the fusible conductor of the substitute fuse was intact. Hence, it would be extremely desirable to provide a way of readily determining whet-her the fusible conductor of an extremely small enclosed electric fuse is intact or has fused to open the circuit. Yet the extreme'ly small size of such a fuse rnakes it impossible to equip that fuse with a window of the usual type. The present invention provides a way of readily determining whether the fusible conductor of an extremely small enclosed electric fuse is intact or has fused to open the circuit, and it does so by providing a Window for that fuse. it is therefore an object of the present invention to provide an extremely small enclosed electric fuse with a window.

Where a fuse must fit within a housing that is approximately one quarter of an inch long and approximately one quarter of an inch in diameter, the components of that fuse must be extremely small. Such small components are hard to handle; and it would be desirable to provide way of facilitating the handling of such small cotnponents. The present invention provides a Way of facilitating the handling of the small components of the fuse of the present invention. Specifically, the present invention provides a relatively large mounting, secures the fuse terminals to that mounting, and then secures the fusable conductor to those terminals. That mounting, those terminals and that fusible conductor constitute a readily-handled and relatively rugged sub-assembly. It is therefore an object of the present invention to provide a relatively large mounting to which the fuse terminals can be secured, and which coacts with said fuse terminals and with a fusible conductor to constitute a readily handled and relatively rugged sub-asembly.

The housing of the fuse provided by the present invention has a thin wall, and a portion of that wall is rendered even thinner to define a shoulder. That shoulder is frusto-conical in configuration; and is performs the dual functions of limiting the extent to which the mounting can be telescopcd into the housing and of centering that mounting relative to that housing. It is therefore an object of the present invention to further reduce the thin wall of the housing of a fuse to form a frusto-conical shoulder that can limit the extent to which the mounting can be telescoped into the housing and that can center that mounting relative to that housing.

Other and further objects and advantages of the present invention should become apparent from an examination of the drawing and accompanying description.

In the drawing and accompanying description a preferred embodiment of the present invention is shown and described but it is to be understood that the drawing and accompanying description are for the purpose of illustration only and do not limit the invention and that the invention will be defined by the appended claims.

In the drawing, H6. 1 is a plan View of one form of small enclosed electric fuse that is made in accordance with the principles and teachings of the present invention,

FIG. 2 is a sectional view through the fuse of FIG. 1, and it is taken along the plane indicated by the line 2--2 in FIG. 1,

FIG. 3 is a view of the bottom of the fuse of FIG. 1, and

PEG. 4 is a cross sectional view of the mounting, the fuse terminals and the fusible conductor of the fuse of FIG. 1.

Referring to the drawing in detail, the numeral It generally denotes the housing for One embodiment of small enclosed electric fuse that is made in accordance with the principles and teachings of the present invention. That housing is made of insulating material, and it is preferably made of .a refractory material; because such material can be strong and can have a high dielectric value. One such material is steatite. The housing 10 is hollow, as at to, and it is cylindrical; and that housing has an inwardly-extending annular flange id at the upper end thereof. That flange defines a circular openi H at the upper end of the housing; and the diameter of that opening is smaller than the inner diameter of the housing iii. The lower end of the housing is formed so it is initially completely open. The wall of the housing 10 is made thin so that hollow center 16 of that housing has a large diameter. The lower portion of the wall of the housing 10 is made even thinner to provide a reduced-thickness portion 18. That reducedthickness portion coacts with the full thickness portion of the wall of housing It) to define a shoulder which is frusto-conical in configuration. The angle of generation of that shoulder is preferably between forty five and seventy degrees.

The numeral 22 denotes a disc of transparent material; and one suitable transparent material is self-extinguishing cellulose acetate. That material is commercially available, and it is sold under the mark Plastacele. The diameter of the disc 22 is larger than the diameter of the opening 12 at the upper end of the housing it), but that diameter is small enough to permit that disc to be telescoped within the housing It The diameter of the disc 22 is preferably large enough to enable that disc to have a press fit with the full thickness portion of the wall of the housing 163. As a result, when the disc 22 is pressed into position where it abuts the under face of the flange 14, that disc will resist movement relative to the housing lltl. That disc will serve as a window for the housing 10. The annular flange 14 will abut, and will overlie, the outer portions of the upper face of the disc 22; and it will thereby prevent the expulsion of that disc through the upper end of the housing it). That flange will thus coact with the press fit between the disc 22 and the inner surface of the full thickness portion of the wall of the housing ill to fixedly lock that disc in position within the housing 10.

The numeral 24 denotes a mounting, in the form of a disc, which has a diameter that is just slightly smaller than the inner diameter of the reduced-thickness portion 1'8 of the wall of the housing Til. That mounting is electrically non-conductive, and it is preferably made of a refractory material; because such a material can be strong and can have a high dielectric value. One such material is steatite. The mounting 24 has a pair of openings 26 therein, and those openings are parallel to the geometric axis of that mounting. Those openings are located on a diameter of the mounting 2d, and they are adjacent the periphery of that mounting. However, the openings 2-6 are spaced inwardly a short distance from the periphery of the mounting 24, so that the outermost portions of those openings will be spaced just a short distance inwardly of the horizontal projection of the full thickness portion of the wall of the housing 10 whenever the mounting is telescoped within that housing.

The openings 26 in the disc 24 accommodate terminals 28; and those terminals are electrically conductive and are circular in cross section. Those terminals are suitably fixed in position within the openings 26 by being crimped, upset or staked, by being cemented to the mounting 24, or by any other suitable artifice. As indicated by FIG. 4, the terminals 28 are initially parts'of a length of wire which has been bent to have a U-shaped configuration. After that wire has been given its U-shaped configuration, the free ends thereof are telescoped into and through the openings 26 in the mounting 24. Those free ends of that Wire extend upwardly above the upper face of the mounting 24 distances approximating the vertical length of the shoulder 20 at the interior of the housing ltl. While the terminals 23 are still part of the U- shaped wire, those terminals are fixedly secured within the openings 26.

A small diameter fusible conductor 30 is secured to the upper ends of the terminals 28 by masses 31 of solder. That solder is high temperature solder, and it will not melt during normal operation of the fuse. The masses 31 of solder provide full electrical connection between the fusible conductor 30 and the terminals 28,

and they hold that conductor fixed relative to the mounting 24. It will be noted that the fusible conductor 30 extends transversely of the axes of the terminals 28.

The engagement between the terminals 28 and the mounting 24 and the engagement between the fusible conductor 3t) and the terminals 28 are such that the mounting 24 and the terminals 28 and the fusible conductor 3% constitute a sub-assembly. That sub-assembly is readily handled and is relatively rugged. Further, the closed end of the U-shaped wire, of which the terminals 28 are a part, constitutes a convenient handle or finger-receiving member. The overall result is that the said subassembly is easily handled despite its very small size.

The mounting 24 can be assemble-d with the housing 10 by moving that mounting toward that housing to telescope the fusible conductor 30 and the upper ends of the terminals 28 through the open lower end of the housing 10, and by continuing to move that mounting toward that housing until the periphery of that mounting engages and is held by the shoulder 20 at the interior of the housing 10. As the mounting engages the shoulder 20, that shoulder will precisely center that mounting relative to the housing 10. As a result, the shoulder 20 performs the dual functions of limiting the extent to which the mounting 24 can be moved toward the disc 22 and of precisely centering that mounting within the housing it). At the time the mounting 24 is centered, the terminals 23 will be spaced inwardly from, and thus not in contact with, the housing 10; and this is desirable. Also, at the time the mounting 24 is in position, the bottom face of the mounting will be disposed a short distance above the level of the bottom of the housing ll).

Once the mounting 24 has been set in position, a sealing material 32 such as an epoxy resin is placed in engagement with the under surface of that mounting. At the time that sealing material is placed in engagement with that under surface of mounting 2d, the housing 10 and the mounting 24 will be inverted, from the position shown by FIG. 2, so that gravity will urge that sealing material against that mounting. The sealing material 32 will also engage and bond to the terminals 28 and to the reduced-thickness portion 13 of the wall of the housing It The sealing material 32 will not be able to flow into the interior of the housing 10 because the mounting 24 will coact with the shoulder 25 to prevent any inflow of that sealing material.

It will be noted that the spacing between the terminals 28 is greater than one half the diameter of the interior 16 of the housing lit). As a result, the present invention provides a large arc gap between the terminals 23. Further it will be noted that the cross sections of the terminals 28 are very much larger than the cross section of the fusible conductor. In one preferred embodiment of the present invention, the diameter of each terminal 28 is four hundredths of an inch, and hence the cross sectional area of each terminal is approximately thirteen ten-thousandths square inches. The diameter of the fusible conductor 30 can range from as large as five thousandths of an inch to as small as one ten thousandth of an inch or smaller. As a result, the cross sectional area of that fusible conduc tor can range from as large as nineteen millionths of a square inch to as small as eight billionths of a square inch or smaller. This means that while the terminals 2% are small, they are very massive relative to the fusible conductor 3th and will cool any are that forms when the fusible conductor 3d opens the circuit. In addition, it will be noted that as an arc would tend to span the gap between the terminals 23, that are would be intermediate two convex surfaces; and those surfaces would simulate spherical arc horns. These various features coact to enable this extremely small fuse to safely open circuits which are carrying two hundred and forty volts.

After the sealing material 32 has set and hardened, the closed end of the wire, of which the terminals 28 are a part, is cut off. The dashed line AA in FIG. 4 shows where that closed end will be cut away. The cutting away of that closed end forms two prongs, as shown by FIGS. 2 and 3. Those prongs can extend into a suitable socket to connect the fuse into the circuit when it is to protect.

To be effective in cooling any arc that forms as the fusible conductor 30 opens the circuit, the terminals 28 should have cross sections that are very much larger than the cross section of that fusible conductor. Specifically, those terminals should have cross sections that are approximately one hundred or more times the cross section of that fusible conductor.

It will be noted that the upper ends of the terminals 28 are not only spaced apart but are spaced from nearby surfaces. As a result, those upper ends will facilitate the heat-induced upward bowing of any are that forms between those upper ends; and such upward bowing will help tend to quench the arc.

Whereas the drawing and accompanying description have shown and described a preferred embodiment of the present invention, it should be apparent to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.

What I claim is:

The method of making an electric fuse that comprises forming a housing with a cylindrical wall that has a frusto-conical shoulder at the interior thereof intermediate a small diameter portion and a larger diameter portion, said larger diameter portion of said housing being adjacent an open end of said housing, forming an insulating mounting so it can be telescoped through said open end of said housing into said larger diameter portion and can then be moved into engagement with said frusto-conical shoulder, said mounting having spaced apart openings therein, forming a conductor so it has a U-shaped configuration, extending the free ends of said conductor through said spaced apart openings in said mounting so said free ends of said conductor project upwardly beyond the upper face of said mounting, fixedly securing said conductor to said mounting, securing a fusible conductor to said free ends of said conductor, telescoping said mounting through said open end of said housing into said larger diameter portion of said housing and then moving said mounting into engagement with said frusto-conical shoulder in said housing to dispose said free ends of said conductor and to place said fusible conductor within said housing, using said frusto-conical shoulder to center said mounting within larger diameter portion of said housing, securing said mounting to said housing by a sealing material, and cutting the closed end of said conductor to provide two terminals.

References Cited by the Examiner UNITED STATES PATENTS 1,605,688 11/1926 Olin et al. 29155.5 X 1,685,382 9/1928 Stuart 29-155.5 2,292,117 8/1942 Grimshaw 29155.5 X 2,742,686 4/1956 Franz 29155.5 2,766,510 10/1956 Heibel 2925.42 3,023,289 2/1962 McAlister 29l55.5

JOHN F. CAMPBELL, Primary Examiner.

WHITMORE A. WILTZ, Examiner.

R. W. CHURCH, Assistant Examiner. 

